Christia grew in up in the northern Greek city of Salonica in the 1990s, with the Bosnian war raging just over the border. “It was in our neighborhood and we discussed it vividly every night over dinner,” she says. The question of ethnicity seized her imagination: Were different peoples doomed to conflict by incompatible identities? Or were the decision-makers in civil wars working on a different calculus from their emotional followers? As a graduate student at Harvard, Christia flew to Afghanistan and tried to turn a dispassionate political scientist’s eye to the question of why warlords behave the way they do.

Christia spent years studying these warlords, the factional leaders in a civil war that broke out in the late 1970s. As a graduate student and later as a professor, she returned to Afghanistan to interview some of the nastiest war criminals in the country. She concluded that culture and identity, while important for their adherents, did not seem to factor into the motives of the warlords themselves, and specifically not in their choices of wartime allies. Despite the powerful rhetoric about ethnic alliances forged in blood, warlords repeatedly flipped and switched sides. They used the same language—about tribe, religion, or ethnicity—whether they were fighting yesterday’s foe or joining him.

If ethnicity, religion, and other markers of identity didn’t matter to warlords, Christia asked, what did? It turns out the answer was simple: power. After studying the cases of Afghanistan, Bosnia, and Iraq in intricate detail, Christia built a database of 53 conflicts to test whether her theory applied more widely. She ran regression analyses and showed that it did: Warlords adjusted their loyalties opportunistically, always angling for the best slice of the future government. It’s not quite as simple as siding with the presumed winner, she says: It’s picking the weakest likely winner, and therefore the one most likely to share power with an ally.

In this model of warlord behavior, the many factions in a civil war are less like Cain and Abel and more like the mafia families in “The Godfather” trilogy. Loyalties follow business interests, and business interests change; meanwhile, the talk about family and blood keeps the foot soldiers motivated. In Bosnia, one Muslim warlord joined forces with the Serbs after the Serbs’ horrific massacre of Muslims at Srebenica, and justified his switch by saying that the central government in Sarajevo was run by fanatics while he represented the true, moderate Islam. In case after case of intractable civil wars—Afghanistan, Lebanon, Iraq, the former Yugoslavia—Christia found similar patterns of fluid alliances.

“The elites make the decision, and then sell it to the people who follow them with whatever narrative sticks,” Christia said. “We’re both Christians? Or we’re both minorities? Or we’re both anti-communist? Whatever sticks.”

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Christia’s work has been received with great interest, though not all her academic colleagues agree with her conclusions. Critics say identity is more important in civil wars than she gives it credit for, and we ignore it at our peril. Roger Petersen, an expert on ethnic war and Eastern Europe who is a colleague of Christia’s at MIT and supervised her dissertation, argues that in some conflicts, identity—ethnic, religious, or ideological—is truly the most important factor. Leaders might make a pact with the devil to survive, but once a conflict heads to its conclusion, irreconcilable conflicts often end with a fight to the death. Communists and nationalists fought for total victory in Eastern Europe’s civil wars, with no regard to their fleeting coalitions of opportunity against foreign occupiers during World War II. More recently, Bosnia’s war only ended after the country had split into ethnically cleansed cantons.

Christia acknowledges that her theory needs further testing to see if it applies in every case. She is currently studying how identity politics play out at most local level in present-day Syria and Yemen.